Charging roll

ABSTRACT

A charging roll includes a core member, a rubber base material disposed around the core member, and a surface layer disposed around the rubber base material. The ten point height of irregularities RZ of a surface of the surface layer is equal to or greater than 3.9 micrometers, and is equal to or less than 6.1 micrometers. The mean spacing between peaks Sm of the surface of the surface layer is equal to or greater than 12.5 micrometers and is equal to or less than 13.5 micrometers.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase Application under 35 U.S.C.371 of International Application No. PCT/JP2020/007349, filed on Feb.25, 2020, which claims priority to Japanese Patent Application No.2019-034888, filed on Feb. 27, 2019. The entire disclosures of the aboveapplications are expressly incorporated by reference herein.

BACKGROUND Technical Field

The present invention relates to charging rolls of image formingapparatuses.

Related Art

Image quality produced by an image forming apparatus such as anelectrophotographic copying machine depends on uniformity of a chargedstate of the photoconductor element, which is affected by the surfaceroughness of a charging roll. JP-A-2015-121769, JP-A-2012-14141 andJP-A-2005-91414 are known as conventional techniques that relate to asurface roughness of charging rolls.

JP-A-2015-121769 describes a technique related to a charging member(charging roll) consisting of an electroconductive support, anelectroconductive elastic layer laminated on the electroconductivesupport, and an electroconductive resin layer laminated as the outermostlayer on the electroconductive elastic layer. The electroconductiveresin layer contains a matrix material and at least one kind ofparticles selected from a group consisting of resin particles andinorganic particles, the particles containing first particles, in whichA is 10 micrometers to 7.0 micrometers, B1/A is 5.0 to 30.0, and S_(m)is 50 micrometers to 400 micrometers, where the thickness of a portionof the electroconductive resin layer formed by the matrix material aloneis A [micrometers], the mean particle diameter of the particles is B1[micrometers], and the inter-particle distance is S_(m) [micrometers].

JP-A-2012-14141 discloses a technique that relates to an image formingapparatus including a positively-charged single-layer typeelectrophotographic photoconductor element; a charging device having acontact-type charging member for charging the surface of thephotoconductor element; an exposure device for exposing the surface ofthe charged image carrier to form an electrostatic latent image on thesurface of the image carrier; a developing device for developing theelectrostatic latent image as a toner image; and a transfer device fortransferring the toner image from the image carrier to a transferobject. The contact-type charging member is a charging roller made ofelectroconductive rubber and has an Asker-C rubber hardness of 62degrees to 81 degrees, and a surface roughness of the charging roller ofthe contact-type charging member has a mean interval S_(m) of 55micrometers to 130 micrometers between surface irregularities, and aten-point height irregularity R_(Z) of 9 micrometers to 19 micrometers.

JP-A-2005-91414 discloses a technique that relates to a charging rollerincluding an electroconductive support, a roll-shapedsemi-electroconductive elastic layer formed on the electroconductivesupport, and a protective layer formed on the surface of thesemi-electroconductive elastic layer. The protective layer is formed byapplication of a coating liquid for forming the protective layercontaining fine particles that prevent adhesion of an external substanceto the protective layer, with the volume average particle diameter ofthe fine particles being refined such that the surface roughness of theprotective layer is equal to or less than 1 micrometer.

An object of JP-A-2015-121769, JP-A-2012-14141 and JP-A-2005-91414 is tocontrol a discharge between the charging roll and the photoconductorelement to make the discharge as uniform as possible, which is achievedby adjusting a surface roughness of the outermost surface of thecharging roll by use of fine particles in the surface layer, to therebyimprove an image quality.

SUMMARY OF THE INVENTION

Demand exists for image forming apparatuses that provide a high imagequality.

The present invention provides a charging roll that reduces imageunevenness.

A charging roll according to the present invention includes a coremember, a rubber base material disposed around the core member, and asurface layer disposed around the rubber base material, with a ten pointheight of irregularities R_(Z) of a surface of the surface layer beingequal to or greater than 3.9 micrometers, and being equal to or lessthan 6.1 micrometers, and with a mean spacing between peaks S_(m) of thesurface of the surface layer being equal to or greater than 12.5micrometers and being equal to or less than 13.5 micrometers. Accordingto this aspect, image unevenness can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an example of an image formingapparatus including a charging roll according to an embodiment of thepresent invention;

FIG. 2 is a cross-sectional view showing an example of the charging rollaccording to an embodiment of the present invention; and

FIG. 3 is a cross-sectional view of a rubber base material and a surfacelayer cut along the axial direction of the charging roll.

DETAILED DESCRIPTION

An embodiment for carrying out the present invention will now bedescribed in detail. In the drawings, the scale is not necessarily toscale, and some dimensions may be exaggerated for illustrative productsor samples.

As shown in FIG. 1, an image forming apparatus according to anembodiment of the present invention includes a photoconductor element 1.Around the photoconductor element 1, a developing section 2, an exposuresection 3, a charging section 4, a transfer section 6, and a cleaningsection 5 are arranged. In the developing section 2, a developing roll20, a regulating blade 21, and a supply roll 22 are disposed, and thetoner 23 is stored. The charging section 4 is provided with a chargingroll 40. The transfer section 6 transfers the toner image onto a sheet60 of paper, which is a recording medium. The toner image transferred bythe transfer section 6 is fixed by a fusing section (not shown).

The cylindrical and rotating photoconductor element 1 and thecylindrical and rotating charging roll 40 are in contact with each otherat the nip 50. Discharge between the photoconductor element 1 and thecharging roll 40 occurs in the region 51 in front of the nip 50 in therotational direction of the photoconductor element 1 and the chargingroll 40 (in some cases, in addition to discharge in the region 51 infront of the nip 50 discharge occurs in the region 52 behind the nip50), whereby the surface of the photoconductor element 1 is charged.Preferably, the charged state of the surface of the photoconductorelement 1 is uniform in both the circumferential direction and the axialdirection of the photoconductor element 1.

FIG. 2 is a cross-sectional view showing an example of a charging rollaccording to an embodiment of the present invention. As shown in FIG. 2,the charging roll 40 includes a core member 401, a rubber base material402 formed on the outer peripheral surface of the core member 401, and asurface layer 403 coated on the outer peripheral surface of the rubberbase material 402. By coating the outer peripheral surface of the rubberbase material 402 with the surface layer 403 having a coatingcomposition that is formed to have a suitable surface roughness, unevendischarge between the photoconductor element 1 and the charging roller40 can be prevented and uniform discharge can be provided to thephotoconductor element 1 such that the developing section 2 adheres tothe surface of the photoconductor element 1 an amount of toner thataccurately corresponds to the latent image formed by the exposingsection 3.

Core Member

The core member 401 can be formed of a material, including, but notlimited to, a metal or resin material having excellent thermalconductivity and mechanical strength, for example, a metal material suchas stainless steel, nickel (Ni), nickel alloy, iron (Fe), magneticstainless steel, and cobalt-nickel (Co—Ni) alloy, or a resin materialsuch as PI (polyimide resin). The structure of the core member 401 isnot particularly limited, and it may be hollow or not hollow.

Rubber Base Material

The rubber base material 402 is disposed on the outer peripheral surfaceof the core member 401, and is formed of electroconductive rubber havingconductivity. The rubber base material 402 may be composed of a singlelayer or two or more layers. In addition, an adhesion layer, anadjustment layer, etc. may be interposed between the core member 401 andthe rubber base material 402, as appropriate.

The rubber base material 402 can be formed by molding a rubbercomposition, which is obtained by adding a conductivity impartingmaterial, a crosslinking agent, etc. to an electroconductive rubber,around the core member 401. Examples of the electroconductive rubberinclude polyurethane rubber (PUR), epichlorohydrin rubber (ECO), nitrilerubber (NBR), styrene rubber (SBR), and chloroprene rubber (CR).

As the conductivity imparting material, an electronic conductivityimparting material such as carbon black or metal powder, an ionicconductivity imparting material, or a mixture thereof can be used.

Examples of the ionic conductivity imparting material include organicsalts, inorganic salts, metal complexes, and ionic liquids. An exampleof an organic salt is sodium trifluoride acetate, and examples of theinorganic salt includes lithium perchlorate and quaternary ammoniumsalt. An example of a metal complex is ferric halide-ethylene glycol,and specific examples thereof include those described in JP-B-3655364.The ionic liquid is a molten salt that is liquid at room temperature,and is referred to as a room temperature molten salt. The salt has amelting point of 70 degrees Celsius or less, preferably 30 degreesCelsius or less. Specific examples thereof include those described inJP-A-2003-202722.

The crosslinking agent is not particularly limited, and examples thereofinclude sulfur and a peroxide vulcanizing agent.

Furthermore, a crosslinking aid, etc. that promotes action of thecrosslinking agent may be added to the rubber composition, asappropriate. Examples of the crosslinking aid include inorganicmaterials, such as zinc oxide and magnesium oxide, and organicmaterials, such as stearic acid and amines. In addition, to shorten atime taken to achieve crosslinking, a thiazole-based or othercrosslinking accelerator may be used. Other additives may be added tothe rubber composition, as appropriate.

In this embodiment, the surface of the rubber base material 402 formedon the outer peripheral surface of the core member 401 is first groundto a predetermined thickness with a grinding machine, after which thesurface of the rubber base material 402 is subjected to dry grindingwith a grinding wheel. The surface layer 403 is then formed on the outerperipheral surface of the rubber base material 402. Grinding isperformed to adjust the surface roughness of the rubber base material402 as appropriate, and to thereby adjust the surface roughness of thesurface layer 403 formed on the outer peripheral surface of the rubberbase material 402.

In a case in which the surface roughness of the rubber base material 402is to be minimized, the surface roughness (ten-point heightirregularities) R_(Z) according to JIS B 0601 (1994) of the rubber basematerial 402 is preferably equal to or less than 8.5 micrometers. Thesurface roughness R_(Z) is measured by a contact-type surface roughnessmeter.

Dry grinding is performed, for example, in a state in which the rubberbase material 402 is rotated, by moving the rotary grinding wheel alongthe axial direction of the core member 401 while the wheel is in contactwith the rubber base material 402 (traverse grinding). In a case inwhich the surface roughness of the rubber base material 402 is to beminimized, the number of revolutions of the grinding wheel of thegrinding machine may be gradually increased, for example, from 1000 rpm,to 2000 rpm, to 3000 rpm. Alternatively, the coarseness of a grindingwheel may be progressively changed. For example, a GC (greencarborundum) grinding wheel may be changed, for example, from a GC 60wheel, to a GC 120 wheel, to a GC 220 wheel.

In addition, after the surface of the rubber base material 402 isdry-ground, the surface may be wet ground with a wet grinding machine inwhich a waterproof grinding paper such as waterproof sandpaper isemployed, with the rubber base material 402 being brought into contactwith the sandpaper under supply of a grinding liquid.

Rubber Hardness of Rubber Base Material

The rubber hardness of the base material 402 is measured by use of adurometer “Type A” according to JIS K 6253 and ISO 7619, and thehardness is preferably within a range from 50 degrees to 64 degrees.

The surface layer 403 formed on the rubber base material 402 is thin,and thus the hardness of the surface of the charging roll 40 is affectedby a hardness of the rubber base material 402. In a case in which thehardness of the rubber base material 402 is less than 50 degrees, convexportions on the surface of the charging roll 40 are likely to be crushedand contaminate the photoconductor element 1, and cause image defects.On the other hand, if the hardness of the rubber base material 402 isgreater than 64 degrees, convex portions on the surface of the chargingroll 40 may affect the image.

Surface Layer

In this embodiment, a coating liquid is applied to the outer peripheralsurface of the rubber base material 402 and dried and cured, therebyforming the surface layer 403. Application of the coating liquid may becarried out by dip coating, roll coating, spray coating, or the like.

As shown in FIG. 3, the cured surface layer 403 includes anelectroconductive matrix 404 and particles 405 of a surface roughnessenhancing material (also referred to as a roughness enhancing material),which may be, e.g., an electric insulator, dispersed in theelectroconductive matrix 404. The particles 405 of the roughnessenhancing material provide the surface layer 403 with an appropriatesurface roughness. The electroconductive matrix 404 serves to hold theparticles 405 of the roughness enhancing material in position and servesto effect discharge to the photoconductor element 1. Theelectroconductive matrix 404 contains a base material and anelectroconductive material dispersed in the base material. As describedabove, discharge occurs between the charging roller 40 and thephotoconductor element 1 in the region 51 (and in some cases in theregion 52, also).

In the example shown in FIG. 3, the particles 405 of the roughnessenhancing material are not completely embedded in the electroconductivematrix 404; however, they may be completely embedded. If the thicknessof the electroconductive matrix 404 is small, the ability of the matrixto hold the particles 405 of the roughness enhancing material will alsobe low. Accordingly, it is preferable for the electroconductive matrix404 to have a thickness that is appropriate relative to the diameter ofthe particles 405 of the roughness enhancing material. When theparticles 405 of the roughness enhancing material are made of anelectric insulator, when the thickness of the electroconductive matrix404 is large, and when the electrical resistance of theelectroconductive matrix 404 is large, discharge is less likely tooccur. However, by increasing the proportion of the electroconductivematerial contained in the electroconductive matrix 404, the electricalresistance of the electroconductive matrix 404 can be reduced tofacilitate occurrence of discharge.

In the present embodiment, the surface roughness of the surface layer403 is adjusted by dispersing the particles 405 of the roughnessenhancing material in the surface layer 403 formed on the rubber basematerial 402, of which the surface roughness is adjusted.

In the present embodiment, it would be preferable for the thickness ofthe electroconductive matrix 404 of the surface layer 403 to be withinan appropriate numerical range. It is contemplated that if the thicknessis too large, the surface roughness of the surface layer 403 will be toosmall resulting in image unevenness.

Furthermore, in the present embodiment, it would be preferable for theamount of the particles 405 of the roughness enhancing material in thesurface layer 403 to be within an appropriate numerical range. It iscontemplated that if the amount of the particles is large, the particlesmay overlap, causing the surface of the surface layer 403 to be rough,and resulting image unevenness.

In this embodiment, the composition of the coating liquid that is thematerial of the surface layer 403 contains at least the base material,the electroconductive material, and the particles 405 of the surfaceroughness enhancing material. After curing of the coating liquid, thebase material and the electroconductive material become components ofthe electroconductive matrix 404.

The coating liquid is obtained, for example, by dissolving in a diluentsolvent the following components.

Composition of Coating Liquid

Base material, 10 to 80 parts by weight;

electroconductive material, 1 to 50 parts by weight; and

surface roughness enhancing material, 70% by weight or less of the totalamount of the coating liquid.

It is contemplated that when the surface roughness and the pitches ofirregularities of the surface layer 403 are appropriate, dischargebetween the charging roll 40 and the photoconductor element 1 will besubstantially uniform in the gap before the nip, at which the chargingroll 40 and the photoconductor element 1 are in contact with each other,so that uneven discharge will not occur upon image formation, whereby animage of a desired density will be formed, with an end result ofprovision of high image quality.

It is considered that the surface roughness and the pitches ofirregularities of the surface layer 403 can be adjusted as appropriateby adjusting the particle diameter and the amount of the particles 405of the surface roughness enhancing material.

Base Material

The base material contained in the coating liquid is an electricinsulator. Examples of the base material include urethane resin, acrylicresin, acrylic urethane resin, amino resin, silicone resin, fluorineresin, polyamide resin, epoxy resin, polyester resin, polyether resin,phenol resin, urea resin, polyvinylbutyral resin, melamine resin, nylonresin, etc. The base materials may be used alone or in combination.

Electroconductive Material

Examples of the electroconductive material contained in the coatingliquid include a carbon black such as acetylene black, Ketjen black, andTokablack, a carbon nanotube, an ion such as lithium perchloride, anionic liquid such as 1-butyl-3-methylimidazolium hexafluorophosphate,and a metal oxide such as tin oxide, and an electroconductive polymer.These electroconductive materials may be used alone or in combination.

Surface Roughness Enhancing Material

Examples of the particles 405 of the surface roughness enhancingmaterial contained in the coating liquid include acrylic particles,urethane particles, polyamide resin particles, silicone resin particles,fluororesin particles, styrene resin particles, phenol resin particles,polyester resin particles, olefin resin particles, epoxy resinparticles, nylon resin particles, carbon, graphite, carbide balloon,silica, alumina, titanium oxide, zinc oxide, magnesium oxide, zirconiumoxide, calcium sulfate, calcium carbonate, magnesium carbonate, calciumsilicate, aluminum nitride, boron nitride, talc, kaolin clay,diatomaceous earth, glass beads, hollow glass spheres, etc. Theseparticles may be used alone or in combination.

Relationship Between Content Percentage and Particle Diameter

It is considered that there is a preferable range with respect to therelationship between the particle diameter and the amount of theparticles 405 of the surface roughness enhancing material in the coatingliquid in order to improve the image quality.

Diluent Solvent

The diluent solvent contained in the coating liquid is not particularlylimited, and examples thereof include an aqueous-based solvent or othersolvents such as methyl acetate, ethyl acetate, butyl acetate, methylethyl ketone (MEK), methyl isobutyl ketone (MIBK), methanol, ethanol,butanol, 2-propanol (IPA), acetone, toluene, xylene, hexane, heptane,and chloroform.

Working Examples

Experiments were conducted to investigate characteristics of thecharging roll 40 suitable for reducing image unevenness.

Experiment 1

In Experiment 1, samples 1-4 of the charging roll 40 according to theembodiment were produced.

Preparation of Rubber Base Material

A rubber composition obtained by adding 0.5 parts by weight of sodiumtrifluoroacetate (as a conductivity imparting material), 3 parts byweight of zinc oxide, 2 parts by weight of stearic acid, and 1.5 partsby weight of a crosslinking agent to 100 parts by weight ofepichlorohydrin rubber (“Epichlomer CG-102” manufactured by Osaka SodaCo., Ltd., Osaka, Japan) was kneaded with a roll mixer.

The kneaded rubber composition was formed into a sheet material andwound around the surface of a core member 401 having a diameter of 6 mm.The sheet material was press-molded to form a rubber base material 402made of crosslinked epichlorohydrin rubber.

The hardness of the resulting rubber base material 402 was measuredusing a durometer “Type A” according to JIS K 6253 and ISO 7619. Themeasured hardness was within a range from 50 degrees to 64 degrees.

Grinding Surface of Rubber Base Material

The surface of the rubber base material 402 was ground with a grindingmachine. More specifically, the surface of the obtained rubber basematerial 402 was ground with a grinding machine to provide the rubberbase material 402 with a predetermined thickness (1.25 mm), followed bydry grinding in which the rotation speed of the grinding wheel of thegrinding machine was gradually increased from 1000 rpm, to 2000 rpm, to3000 rpm. That is, in Experiment 1, the surface roughness of the rubberbase material 402 was minimized.

Preparation of Coating Liquid

A coating liquid for forming the surface layer 403 on the outerperipheral surface of the rubber base material 402 described above wasprepared.

Composition of Coating Liquid

The composition of the coating liquid is as shown in Table 1.

TABLE 1 Composition of Coating Liquid Ratio Material (Parts by FunctionMaterial Details Weight) Diluent Ethyl Acetate 60.0 Solvent BaseUrethane Resin 19.9 Material Contents of Base Material Polyol “T5650E”Manufactured by Asahi Kasel Chemicals Corp. 10.8 (Tokyo, Japan)lsocyanurate “TPA-100” Manufactured by 9.1 Asahi Kasei Chemicals Corp.(Tokyo, Japan) Electro- Carbon Dispersed “MHI-BK” (Including 20 to 3018.4 conductive Liquid Weight % of Carbon) Material Manufactured byMikuni Color Ltd. (Hyogo, Japan) Additive Acrylic Silicone “MODIPERFS-700” 1.0 Polymer Manufactured by NOF Corp. (Tokyo, Japan) SurfaceUrethane Particles Urethane Beads Manufactured See Roughness by NegamiChemical Industrial Table 2 Enhancing Co., Ltd. (Tokyo, Japan) Material

Urethane beads manufactured by Negami Chemical Industrial Co., Ltd.(Tokyo, Japan) were used as the urethane particles.

The relationship between the average particle diameter of the urethanebeads and the product name used in Experiments 1 and 2 is as follows. Itis of note that in practice, one product contains particles havingdiameters that differ from the average particle diameter.

6 micrometers: Urethane beads “C-800”

10 micrometers: Urethane beads “C-600”

15 micrometers: Urethane beads “C-400”

In Experiment 1, samples 1-4 having different surface conditions of thesurface layer 403 were produced by applying coating liquids containingparticles 405 of the surface roughness enhancing material havingdifferent particle diameters and in different amounts. The surfaceroughness and the pitches of irregularities of the surface layer 403 inthe samples are as shown in Table 2.

The coating liquid having the above composition was stirred with a ballmill for 3 hours.

TABLE 2 Surface Roughness Enhancing Material R_(z) of S_(m) of AverageContent Image Unevenness Image Surface Surface Particle Percentage InLocal Comprehensive Layer Layer Diameter Coating Liquid DischargeScumming Judgment [μm] [μm] [μm] [wt %] — — — Sample 1 3.9 13.0 10 2Good Good Good Sample 2 6.1 13.0 6 2 Good Good Good Sample 3 2.2 10.5 102 Bad Bad Bad Sample 4 2.5 16.1 15 2 Bad Bad Bad Sample 5 9.8 21.2 6 15Bad Good Bad

Preparation of Charging Roll

The surface layer 403 was formed by applying the coating liquid to theouter peripheral surface of the ground rubber base material 402, tomanufacture a charging roll 40. Specifically, the coating liquid wasstirred, and the liquid was spray-coated on the surface of the rubberbase material 402, and dried in an electric furnace at 120 degreesCelsius for 60 minutes to form the surface layer 403 on the outerperipheral surface of the rubber base material 402, to produce a chargedroll.

Measurement of Surface Roughness R_(Z)

Using a contact-type surface roughness measuring machine (Surf Coder“SE500” manufactured by Kosaka Laboratory Ltd. (Tokyo, Japan)), thesurface roughness (ten point height of irregularities) R_(Z) (conformingto JIS B 0601: 1994) of the surface layer 403 was measured under themeasurement conditions described below. The measurement results areshown in Table 2.

Measurement Conditions

Cutoff: λc=0.8 mm

Measurement length: 4 mm (sampling length method)

Measurement speed: 0.5 mm/sec

Measurement positions: Surface roughness R_(Z) was measured at 3 pointsof each charging roll 40. Then, the average of these values wascalculated. The average value is shown in Table 2.

Measurement of Mean Spacing Between Peaks S_(m)

The mean spacing between peaks S_(m) at the surface of the surface layer403 (conforming to JIS B 0601:1994) was measured.

First, two portions of the surface of the central part in the axialdirection of the charging roll 40 was photographed with a non-contacttype laser microscope. The laser microscope used was a “VK-X200”manufactured by Keyence Corporation (Osaka, Japan). Magnification was400 times, and the photographic field of view of each of the twoportions was 528.7 micrometers along the circumferential direction ofthe charging roll 40 and was 705.1 micrometers along the axial directionof the charging roll 40.

Next, using Version 1 2.0.116 of the multi-file analysis application“VK-H1XM” produced by Keyence Corporation, the value S_(m) wascalculated for 19 lines at equal intervals in each field of viewphotographed. Each line was along the axial direction. The cutoff valueλc was 0.8 mm. Furthermore, these values S_(m) were averaged. Table 2shows the average of values S_(m).

Evaluation of Image Unevenness and Discharge Unevenness

An image evaluation test of the samples of the charging roll wasconducted using a copying machine. The copying machine was a colormultifunction peripheral (MFP) “bizhub C3850” (DC-voltage supply type)manufactured by Konica Minolta Inc. (Tokyo, Japan).

The applied charging voltage was measured with a tester. In Experiment1, a voltage (REF−100 V), which was 100 V lower than the normal voltage(REF), was applied by way of an external power supply.

The charging roll was applied to the copying machine, and imageunevenness was evaluated for images (halftone images and white solidimages) printed under the conditions described below. The results areshown in Table 2.

For the image unevenness evaluation, occurrence of local discharge wasjudged on the basis of the halftone images, and lightness was judged onthe basis of the white solid images. Occurrence of local discharge wasconfirmed by visual detection of white spots, black spots, whitestreaks, or black streaks in the halftone images.

Printing Conditions

Speed: 38 sheets/minute

Printing environment: The temperature was 23 degrees Celsius and thehumidity was 55%.

Local Discharge Evaluation

For the halftone images, occurrence of image unevenness caused by localdischarge was evaluated by visual observation using the followingcriteria.

Good: No image unevenness caused by local discharge.

Bad: Image unevenness caused by local discharge.

Lightness Determination

The L* value (lightness) was measured at seven points in each solidwhite image by a chroma meter, “CR-400” manufactured by Konica MinoltaInc. The lightness was evaluated with the following evaluation criteria.The reason why the lightness was measured was to determine whetherscumming, i.e., fogging (printing on a non-print area) occurred.

Evaluation Criteria

Good: No scumming. (L* is 95.5 or more)

Bad: Scumming. (L* is lower than 95.5)

Samples in which image unevenness occurred due to local discharge orscumming were judged to be bad in image comprehensive judgment, andsamples in which image unevenness or scumming did not occur were judgedto be good in image comprehensive judgment. These were described inTable 2.

Experiment 2

In Experiment 2, sample 5 of the charging roll 40 according to theembodiment was produced.

Preparation of Rubber Base Material

The rubber base material was the same as that in Experiment 1.

Grinding Surface of Rubber Base Material

The surface of the rubber base material 402 was ground with a grindingmachine. More specifically, the surface of the obtained rubber basematerial 402 was ground with a grinding machine to provide the rubberbase material 402 with a predetermined thickness (2 mm), after which drygrinding was applied. In Experiment 2, the rotation speed of thegrinding wheel was not changed.

Preparation of Coating Liquid

A coating liquid for forming the surface layer 403 on the outerperipheral surface of the rubber base material 402 described above wasprepared.

Composition of Coating Liquid

The composition of the coating liquid is shown in Table 1.

Urethane beads manufactured by Negami Chemical Industrial Co., Ltd.(Tokyo, Japan) were used as the urethane particles.

In Experiment 2, sample 5 was produced by applying coating liquidscontaining particles 405 of the surface roughness enhancing materialhaving the particle diameter and the amount in Table 2. The surfaceroughness and the pitches of irregularities of the surface layer 403 insample 5 are as shown in Table 2.

The coating liquid having the above composition was stirred with a ballmill for 3 hours.

Preparation of Charging Roll

In the same manner as that of Experiment 1, the surface layer 403 wasformed by applying the coating liquid to the outer peripheral surface ofthe ground rubber base material 402, to manufacture a charging roll 40.

Measurement of Surface Roughness R_(Z)

Using the same procedure as in Experiment 1, the surface roughness (tenpoint height of irregularities) R_(Z) on the surface of the surfacelayer 403 was measured. The ten point height of irregularities R_(Z) isshown in Table 2.

Measurement of Mean Spacing Between Peaks S_(m)

Using the same procedure as in Experiment 1, the mean spacing betweenpeaks S_(m) at the surface of the surface layer 403 (conforming to JIS B0601:1994) was measured.

Evaluation of Image Unevenness and Discharge Unevenness

An image evaluation test of the samples of the charging roll wasconducted using a copying machine. The copying machine was a colormultifunction peripheral (MFP) “MP C5503” (AC/DC voltage-superimposedsupply type) manufactured by Ricoh Company, Ltd. (Tokyo, Japan).

The DC voltage was the normal voltage (REF), and the AC voltage V_(pp)was controlled by the copying machine.

In Experiment 2, the alternating current was set at 1.40 mA, which islower than the normal alternating current (REF) of the copying machine.

The charging roll was applied to the copying machine, and the imageunevenness was evaluated for images (halftone images and white solidimages) printed under the following printing conditions. The results areshown in Table 2. For the image unevenness evaluation, occurrence oflocal discharge was judged on the basis of the halftone images.Occurrence of local discharge was confirmed by visual detection of whitespots, black spots, white streaks, or black streaks in the halftoneimages. Occurrence of scumming, i.e., fogging was judged by visualdetection in the white solid images.

Printing Conditions

Speed: 30 sheets/minute

Printing environment: The temperature was 23 degrees Celsius and thehumidity was 55%.

Local Discharge Evaluation

For the halftone images, occurrence of image unevenness caused by localdischarge was judged by visual observation using the following criteria.

Good: No image unevenness caused by local discharge.

Bad: Image unevenness caused by local discharge.

Determination of Scumming

For the white solid images, occurrence of scumming, i.e., fogging(printing on a no-print area) was judged by visual observation.

Evaluation Criteria

Good: No scumming.

Bad: Scumming.

Although scumming did not occur in sample 5, image unevenness caused bylocal discharge occurred in sample 5. Accordingly, sample 5 was judgedto be bad in image comprehensive judgment.

As will be apparent from Table 2, it is preferable for the ten pointheight of irregularities R_(Z) of the surface of the surface layer 403to be equal to or greater than 3.9 micrometers and to be equal to orless than 6.1 micrometers, and it is preferable for the mean spacingbetween peaks S_(m) of the surface of the surface layer 403 to be 13.0micrometers. However, considering the measurement error, it ispreferable for the mean spacing between peaks S_(m) of the surface ofthe surface layer 403 to be equal to or greater than 12.5 micrometersand to be equal to or less than 13.5 micrometers.

The invention claimed is:
 1. A charging roll comprising: a core member; a rubber base material disposed around the core member; and a surface layer disposed around the rubber base material, a ten point height of irregularities R_(Z) of a surface of the surface layer being equal to or greater than 3.9 micrometers and being equal to or less than 6.1 micrometers, and a mean spacing between peaks S_(m) of the surface of the surface layer being equal to or greater than 12.5 micrometers and being equal to or less than 13.5 micrometers, wherein the surface layer comprises an electroconductive matrix comprising a base material formed an electric insulator and an electroconductive material being dispersed in the base material, and particles of a surface roughness enhancing material being dispersed in the electroconductive matrix, wherein the base material is formed of urethane resin, and wherein the particles of the surface roughness enhancing material are formed of urethane.
 2. The charging roll according to claim 1, wherein the rubber base material has a ten point height irregularities Rz equal to or less than 8.5 micrometers. 